The present invention relates to semiconductor devices and their methods of fabrication. More particularly, the present invention relates to the processing of copper interconnect material and the resultant device utilizing the same. Even more particularly, the present invention relates to reducing electromigration in copper interconnect lines by doping their surfaces with barrier material using wet chemical methods.
Currently, the semiconductor industry is demanding faster and denser devices (e.g., 0.05-xcexcm to 0.25-xcexcm) which implies an ongoing need for low resistance metallization. Such need has sparked research into resistance reduction through the use of barrier metals, stacks, and refractory metals. Despite aluminum""s (Al) adequate resistance, other Al properties render it less desirable as a candidate for these higher density devices, especially with respect to its deposition into plug regions having a high aspect ratio cross-sectional area. Thus, research into the use of copper as an interconnect material has been revisited, copper being advantageous as a superior electrical conductor, providing better wettability, providing adequate electromigration resistance, and permitting lower depositional temperatures. The copper (Cu) interconnect material may be deposited by chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), sputtering, electroless plating, and electrolytic plating.
However, some disadvantages of using Cu as an interconnect material include etching problems, corrosion, and diffusion into silicon.1 These problems have instigated further research into the formulation of barrier materials for preventing electromigration in both Al and Cu interconnect lines. In response to electromigration concerns relating to the fabrication of semiconductor devices particularly having aluminum-copper alloy interconnect lines, the industry has been investigating the use of various barrier materials such as titanium-tungsten (TiW) and titanium nitride (TiN) layers as well as refractory metals such as titanum (Ti), tungsten (W), tantalum (Ta), and molybdenum (Mo) and their silicides.2 Although the foregoing materials are adequate for Al interconnects and Al-Cu alloy interconnects, they have not been entirely effective with respect to all-Cu interconnects. Further, though CVD has been conventionally used for depositing secondary metal(s) on a primary metal interconnect surface, CVD is not a cost-effective method of doping Cu interconnect surfaces with calcium (Ca) ions.
1Peter Van Zant, Microchip Fabrication: A Practical Guide to Semiconductor Processing, 3rd Ed., p. 397 (1997). 
2Id., at 392. 
Some related art methods for forming metal oxide films as barrier materials generally include two electrochemical approaches: (1) electrochemical oxidation using an external electrical current (e.g., La1xe2x88x92xSrxMnO3 and LaCoO3); and (2) electroless deposition (i.e., that which does not require an electrical current to drive the reaction (e.g., PbO2, MnO2, Tl2O3, NiO, LaMnO3). Therefore, a need exists for a low cost and high throughput method of forming a Cuxe2x80x94Caxe2x80x94O thin film on a Cu surface in a chemical solution which improves interconnect reliability, enhances electromigration resistance, and improves corrosion resistance.
Accordingly, the present invention provides a method for forming a Cuxe2x80x94Caxe2x80x94O thin film on a Cu surface by immersing the Cu surface into a unique chemical (electroless plating) solution containing salts of calcium (Ca) and copper (Cu), their complexing agents, a reducing agent, a pH adjuster, and surfactants; and a semiconductor device thereby formed. The present invention further provides a particular method which controls the parameters of pH, temperature, and time in order to form a uniform conformal Cu-rich Cuxe2x80x94Caxe2x80x94O thin film for reducing electromigration in Cu interconnect lines by decreasing the drift velocity therein which decreases the Cu migration rate in addition to decreasing the void formation rate. The Cuxe2x80x94Caxe2x80x94O thin film may also comprise incidental contaminants such as carbon (C) and sulphur (S).
More specifically, the present invention provides a method for forming a semiconductor device having a copper-calcium-oxide (Cuxe2x80x94Caxe2x80x94O) thin film on a copper (Cu) surface, the Cu surface having been formed by CVD, PVD, or electroplating, by treating the Cu surface in a chemical solution, comprising the steps of: (1) providing the chemical solution, the chemical solution comprising: (a) at least one calcium (Ca) ion source for providing a plurality of Ca ions; (b) at least one complexing agent for complexing the plurality of Ca ions; (c) at least one copper (Cu) ion source for providing a plurality of Cu ions; (d) at least one complexing agent for complexing the plurality of Cu ions; (e) at least one pH adjuster; (f) at least one reducing agent for facilitating deposition of the plurality of Cu ions; (g) at least one wetting agent for stabilizing the chemical solution; and (h) a volume of water, (a) through (g) being dissolved in (h); (2) immersing the Cu surface in said chemical solution, thereby forming the Cuxe2x80x94Caxe2x80x94O thin film on the Cu surface; and (3) rinsing the Cuxe2x80x94Caxe2x80x94O thin film formed on the Cu surface in water; drying the the Cuxe2x80x94Caxe2x80x94O thin film formed on the Cu surface under a gaseous nitrogen flow (GN2); and completing formation of the semiconductor device. By forming this Cuxe2x80x94Caxe2x80x94O thin film on the Cu surface, the present invention improves Cu interconnect reliability, enhances electromigration resistance, and improves corrosion resistance.